1. Field of the Invention
The invention relates in general to a network device, and more particularly to a network device having a transmitting unit with correction function.
2. Description of the Related Art
Due to the advancing technology, the application areas of network have become broader and broader and thus the network has become an indispensable part in the daily life. In view of the growing demands (e.g., on-line multimedia application) on the network bandwidth, the speed of the network device has to be increased from the conventional 10/100 MHz to 1 GHz. Taking the high-speed Ethernet with the transmission speed of 1 GHz as an example, each connection port of the network device has four channels, and each channel has four I/O pads to be electrically connected to a primary side of a transformer and an impedance-matching resistor. The transformer is for filtering out DC components of the received or outputted signal. The secondary side of the transformer is coupled to twisted lines of the Ethernet in order to transmit/receive differential transmission signals. The network device simultaneously uses four channels when it performs the transmitting and receiving operations with another network device, and each channel performs the transmitting and receiving functions simultaneously.
FIG. 1 is a schematic illustration showing a conventional voltage mode network device. The network device includes a transmitting unit 10 and a receiving unit 12. The transmitting unit 10 includes a voltage-controlled current source 18, a line driver 16, and feedback resistors Rf1 and Rf2. The voltage-controlled current source 18 is for receiving a digital differential signal S and outputting a corresponding differential current according to the digital differential signal S. The line driver 16 is a differential operational amplifier having a first input terminal, a second input terminal, a first output terminal and a second output terminal. The differential operational amplifier 16 amplifies the differential current outputted from the voltage-controlled current source 18 and outputs differential output signals Vo1 and Vo2 from the first output terminal and the second output terminal, respectively. The differential output signals Vo1 and Vo2 are fed back to the first input terminal and the second input terminal through the feedback resistors Rf1 and Rf2, respectively. Two impedance-matching resistors Rm1 and Rm2, which are respectively electrically connected to the output terminals of the line driver 16, provide for matching function with respect to the equivalent impedance ZL of the twisted lines. The equivalent impedance of the typical twisted lines is 100Ω, and the impedances of the impedance-matching resistors Rm1 and Rm2 are 50Ω. The line interface 14 is coupled to the impedance-matching resistors Rm1 and Rm2. The line interface may be a transformer 14 for filtering out DC components of the received or outputted signals. The I/O pads P are electrically connected to a primary side of the corresponding transformer 14, and a secondary side of the transformer 14 is coupled to the twisted lines. Because the impedance-matching resistor Rm1, the equivalent impedance ZL of the twisted lines, and the other impedance-matching resistor Rm2 form a voltage divider, the peak-to-peak value of the output signal of the line driver 16 is (Vo1−Vo2), the peak-to-peak value of each of the differential transmission signals Tx+ and Tx− is only one half that of (Vo1−Vo2), i.e., (½)*(Vo1−Vo2).
In addition, because the twisted lines simultaneously receive and transmit signals, the differential received signal is coupled to the output differential transmission signal at the input terminal, but is not coupled to the output signals Vo1 and Vo2 of the line driver 16. The receiving unit 12 may then subtract the coupled transmission differential signals Tx+ and Tx− from the received coupled differential signal according to the signals Vo1 and Vo2 outputted from the differential operational amplifier, and thus the differential received signal can be obtained.
However, owing to the reasons such as the process variation, the properties and values of various devices, such as resistors, current sources, or the like, within the IC cannot be precisely controlled. The conventional solution is to use precision resistors outside the IC as the impedance-matching resistors Rm1 and Rm2, and thus two additional I/O pads are needed to input the received differential signal to the receiving unit 12.